From the USPS website
Post Offices Close Early on Christmas Eve and New Year’s Eve.
Post Offices nationwide will be open on Christmas Eve, Monday, Dec. 24, and New Year’s Eve, Monday, Dec. 31. Most Post Offices will shorten retail lobby hours and close at noon on these holiday eves. Regular mail delivery for Dec. 24 and 31 will be unaffected by the change.
Revised hours will be posted at each Post Office. If you are a business customer, please check with your Bulk Mail Entry Unit for specific information regarding holiday hours of operation.
Post Offices will be closed Tuesday, Dec. 25 and Tuesday, Jan. 1. Only Express Mail® will be delivered on Christmas Day and New Year’s Day. All Post Offices will be open and regular mail delivery will resume on Wednesday, Dec. 26 and Wednesday, Jan. 2.